Field
The present embodiment relates to the measurement of propulsive power delivered by vehicles through the wheels while mounted inside a wind tunnel.
Description of Prior Art
Inherently characteristic of rotating vehicle wheels, and particularly of spoked wheels, aerodynamic resistance, or parasitic drag, is an unwanted source of energy loss in propelling a vehicle. Parasitic drag on a wheel includes viscous drag components of form (or pressure) drag and frictional drag. Form drag on a wheel generally arises from the circular profile of a wheel moving though air at the velocity of the vehicle. The displacement of air around a moving object creates a difference in pressure between the forward and trailing surfaces, resulting in a drag force that is highly dependent on the relative wind speed acting thereon. Streamlining the wheel surfaces can reduce the pressure differential, reducing form drag.
Frictional drag forces also depend on the speed of wind impinging exposed surfaces, and arise from the contact of air moving over surfaces. Both of these types of drag forces arise generally in proportion to the square of the relative wind speed, per the drag equation. Streamlined design profiles are generally employed to reduce both of these components of drag force.
The unique geometry of a wheel used on a vehicle includes motion both in translation and in rotation; the entire circular outline of the wheel translates at the vehicle speed, and the wheel rotates about the axle at a rate consistent with the vehicle speed. Form drag forces arising from the moving outline are apparent, as the translational motion of the wheel rim must displace air immediately in front of the wheel (and replace air immediately behind it). These form drag forces arising across the entire vertical profile of the wheel are therefore generally related to the velocity of the vehicle.
As the forward profile of a wheel facing the direction of vehicle motion is generally symmetric in shape, and as the circular outline of a wheel rim moves forward at the speed of the vehicle, these form drag forces are often considered uniformly distributed across the entire forward facing profile of a moving wheel (although streamlined cycle rims can affect this distribution somewhat). This uniform distribution of pressure force is generally considered centered on the forward vertical wheel profile, and thereby in direct opposition to the propulsive force applied at the axle, as illustrated in FIG. 9.
However, as will be shown, frictional drag forces are not uniformly distributed with elevation on the wheel, as they are not uniformly related to the speed of the moving outline of the wheel rim. Instead, frictional drag forces on the wheel surfaces are highly variable and depend on their elevation above the ground. Frictional drag must be considered separate from form drag forces, and can be more significant sources of overall drag on the wheel and, as will be shown, thereby on the vehicle.
Vehicles having wind-exposed wheels are particularly sensitive to external headwinds reducing propulsive efficiency. Drag force on exposed wheels increases more rapidly on upper wheel surfaces than on vehicle frame surfaces, causing a non-linear relation from rising wind speeds between net drag forces on vehicle frame surfaces versus net drag forces on vehicle wheel surfaces.
Since upper wheel surfaces are moving against the wind at more than the vehicle speed, the upper wheel drag forces contribute more and more of the total vehicle drag as external headwinds rise. Thus, as external headwinds rise, a greater fraction of the net vehicle drag is shifted from vehicle frame surfaces to upper wheel surfaces.
And these upper wheel drag forces must be overcome by a propulsive counterforce applied at the axle. Such propulsive counterforces suffer a mechanical disadvantage against the upper wheel drag forces, since each net force is applied about the same pivot point located at the bottom where the wheel is in stationary contact with the ground. This mechanical advantage that upper wheel drag forces have over propulsive counterforces further augments the effective vehicle drag that exposed wheels contribute under rising headwinds.
As a result of these magnified effects of upper wheel drag on resisting vehicle propulsion, a simple measure of net drag force on the entire vehicle is inadequate to properly account for the loss in vehicle propulsive efficiency under rising external headwinds. Hence, another method must be employed to measure the actual power output from a vehicle when exposed to headwinds.
Instrumentation commonly used to measure drag forces on vehicles mounted inside a wind tunnel have been designed primarily for use on aircraft, rather than terrestrial vehicles having wheels. Aircraft models are typically suspended on a pedestal mount, with instrumentation designed to measure the total force exerted on the pedestal when the model is subjected to wind-induced drag forces. By measuring the difference in force exerted on the pedastal mount between with and without the model supended thereon, the net drag force exerted on the model alone can be determined.
Instrumentation similar to that used on pedestals for aircraft models has also been adapted for use on terrestrial vehicles mounted inside wind tunnels. The total force exerted on the vehicle from drag is often measured as the net force exerted on the pedastal. From this measurement, and from the apparent speed and weight of the vehicle—which is often immobilized on a running floor that itself is mounted on a pedastal mount—the power dissipated in drag on the vehicle can be determined, absent other frictional losses. This power measurement is assumed to accurately reflect the vehicle internal propulsive power needed to overcome drag forces.
This assumption can be somewhat valid for vehicles with wheels that are entirely shielded from oncoming headwinds. However, vehicles with wind-exposed wheels are particularly sensitive to external headwinds reducing their propulsive efficiency. Wheel drag forces in particular, are greatly magnified against propulsive counterforces—especially when external headwinds are also present—causing substantial inaccuracies from using the direct force-measurement methods of determining overall vehicle drag. A simple measure of net drag force on the frame of the vehicle is inadequate to properly account for the loss in propulsive efficiency of the vehicle. Hence, another method must be employed to measure the actual power output of the vehicle when exposed to headwinds. Nevertheless, these direct force-measurement methods are still employed in the art.
For example, in U.S. Pat. No. 7,360,443 B2, U.S. Pat. No. 7,614,291 B2 and U.S. Pat. No. 8,272,258 B2, force-measuring means are used to directly measure forces parallel to the vehicle motion which act against one or more platforms upon which the vehicle is mounted inside the wind tunnel—the platforms being separated and isolated from the stationary environment. The platforms include the force measuring instrumentation, intended to accurately measure the total drag on the vehicle. However, this arrangement does not accurately reflect the propulsive counterforce needed to overcome drag forces on vehicles having headwind-exposed wheels.
And in patent EP 1 388 881 A1, a dynamometer is employed on the powered wheel of the vehicle, while employing a road-simulating moving belt beneath the vehicle in order to simulate air flow conditions under the vehicle while operating at speed. The arrangement is intended to mimic air flow road conditions for the purpose of emissions measurement from the vehicle. However, all wheels of the vehicle must be simultaneously coupled to the dynamometer in order to obtain an accurate measurement of the effective drag force on a vehicle. And in the arrangement of this art, the vehicle is immobilized on the dynamometer. In order to obtain an accurate measurement of the effective drag force, the vehicle must also instead remain necessarily unconstrained in the direction of vehicle motion, able to freely self-propel on the dynamometer as it does while driving on an actual road.
Finally, in U.S. Pat. No. 7,360,443 B2 and U.S. Pat. No. 7,614,291 B2, the vehicle is also immobilized, being fixed to the supporting weighing plate.